Utricularia Photosynthetic Pathways

Apr 13, 2015
Los Angeles, CA
Hey all. I was discussing the possibility that Utricularia section Orchidioides are CAM plants with @bluemax and he suggested that I test out the theory with a microscope. Great idea. Here's a primer on CAM plants in case you aren't familiar:
I was doing some research the other day and I came to the realization that many epiphytes are CAM plants. There are several different photosynthetic pathways that are beneficial in different situations (C3, C4, CAM). In the majority of plants (C3, C4), the stomata open during the day. (Stomata are organs on the underside of leaves which can open to facilitate gas exchange.) This allows the plants to take in CO2 and release O2 during the day while they are doing photosynthesis. This is good as it means that photosynthesis can occur at a faster rate. This is bad in certain circumstances as water evaporates out of the stomata - a process called transpiration. For context, 97% of water taken up through the roots is lost to transpiration in C3 plants. That's really bad if you're living on a tree in the dry season. CAM plants open their stomata at night in order to reduce the water loss that occurs during the day.
At the moment I have only investigated U. alpina; I'll likely continue to post additional findings for other species here. It's really easy to prepare a slide if you want to try this yourself (although you will likely lose a leaf in the process). I painted clear nail polish in a 1 cm square of the back of a leaf and allowed it to dry for 15 minutes. I then removed the dried nail polish with some clear tape and taped it on a slide. And that's it. I did this during the middle of the day and during the middle of night. The attached photos are at 1000x magnification.

Here's a photo of U. alpina guard cells (which control whether the stomata is open or closed) during the day. The guard cells are pretty straight and there isn't an opening between them.
PXL_20210408_180742471.MP (1).jpg

And here's a photo of the same leaf at night. The guard cells are much rounder and you can see the opening for the stomata.
PXL_20210408_180431096.MP (1).jpg

Hardly a peer reviewed paper - but enough for me!

So, what does this mean for the cultivation of these plants? Well, in order for a plant to be happy it needs to have access to CO2 and to get rid of O2. These plants do most of that gas exchange at night. And, it turns out that stomatal conductance (how much gas can pass through the stomata) depends on a few factors that we can control. Here's a paper about that, if you're interested in doing some further digging. Based on this paper, here are the external factors that control stomatal conductance.
  • Humidity. This one is the one tidbit that's new; the important finding. Because these plants are epiphytic, their stomatal conductance is especially sensitive to relative humidity. Because of this, I try to modulate the humidity in the terrarium over the course of the day. At night I'm regularly fogging with chilled water (which helps drop the temp as well) and keep the humidity at 90-95%. During the day a fan vents out air to drop the humidity down to 45-55%.
  • Light - maybe, we aren't sure. But, I keep these plants under brighter light for a shorter photoperiod (which also happens to be the photoperiod that they receive in nature). I'm guessing that a long photoperiod for these plants will not be terribly beneficial as they will run out of CO2 and build up O2 over the course of the day.
  • Temperature. Low nighttime temps lead to better conductance, usually between 15-25C. So, I grow these plants with a daytime high of 27C and a night time temp of 17C (at least on one side of my terrarium, I'm still working on keeping the temperature more consistent across the length of my terrarium). This is already aligned with how most people grow these plants.
  • Leaf water potential. Without any water, the guard cells don't open. Read: plants need water. Nothing new here. I keep these plants in media which drains well but is always moist, a mix of live long fibered sphagnum and pumice.
Anyway, I hope this was interesting and helpful for the cultivation of these plants. Thanks for coming to my TED talk.
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War. War never changes.
Staff member
Apr 18, 2004
Pasadena, CA
Very cool! I love to see this level of discourse and follow through.

The only caveat I can think of is to be careful of keeping your RH below the dew point. Stomata are primarily on the underside of leaves, but a film of water greatly reduces gas diffusion. Honestly, probably very low on the list of concerns though.

Now to just measure intracellular PEP concentrations to establish optimal photoperiod cycling. ;)